Solved! ASUS X556UA don't recognize new RAM

Dec 3, 2018

I have a laptop ASUS X556UA. Exactly: i5-6200U, Intel graphics 520, W10 Home 64bits.
Memory RAM is DDR3 and it has 4GB non-removable.
I wanted a little RAM upgrade so I bought this: Hynix 4GB DDR3 1333MHz

I plugged in it but neither PC nor CPU-Z recognize the new RAM.
I've searched on the Internet and I tried some options: plug in and out the RAM several times, updated BIOS (to version 405) from ASUS webpage and I disabled fast boot in BIOS.
RAM isn't still being recognized.
I tried Crucial test too and it says "Maximum Memory: 12GB".

I'm not sure what's happening. Maybe RAM stick is wrong?
Any solution or help will be appreciated.

PS: sorry about my English :D
You should've looked for DDR3L-1600MHz rams. Ideally you could've looked for an 8GB stick of DDR3L-1600MHz rated to run at 1.35v.